Verifying the geometries of airfoil castings is required to detect deviations introduced during the casting process. A hard tool creates a very dimensionally accurate wax pattern but remainder of the investment casting process introduces geometric variability into the part. This deviation is inherent in the process and, although it can be controlled, it cannot be completely removed. Although this deviation is present, to some extent, on all castings, parts having high aspect ratio airfoils are particularly susceptible due to the relatively long, slender nature of the airfoils, which become easily distorted. The two types of geometric variation that are commonly introduced are form and positional. Form tolerance refers to the geometry of a particular feature or group of features. Positional tolerance measures the geometric relation between feature(s), that is, the location of the feature(s) in relation to each other. Casting deviations are usually a combination of these two types of variation.
Referring to FIGS. 1-5, the current datum system used for a part 10 with an airfoil 12 is established with six nest points 14, 16, 18, 20, 22, 24, five on the airfoil (14, 16, 18, 20, 22) and one on a radial surface (24) of the platform or shroud; all located within the S-plane 26 (axial plane), T-plane 28 (circumferential or tangential plane) and U-plane 30 (radial plane) (See FIGS. 1 and 2). This datum system is used for measuring all surfaces of the part 10 with respect to the part's nominal geometry. The parts are then inspected and the deviations of the profile and form are determined. When deviations are quite small relative to the intended tolerance there are no issues with the current datum system. However, as deviations increase with respect to the intended tolerances(s), more information is necessary to determine whether the resultant part features are still acceptable.
With the airfoil nest, features at the tip or shroud 32 and root 34 of the part can have significant positional variation due to distortion of the airfoil during casting (See FIGS. 3-5). For instance, solidification induces stresses in the mold, which create distortion in the finished part. This is an unavoidable result of the investment casting process. The form of the features may still be dimensionally correct even if they are out of position. Under the current datum system, the form deviation of the shroud 32 or root 34 may be measured according to a desired virtual form 36 as known to one of ordinary skill in the art. As shown, the desired virtual form 36 may be formed about the shroud 32 or root 34 (FIG. 3) and then expanded to account for distortion (FIG. 4). However, when a tip, shroud or root feature is measured with respect to the airfoil nest in the current datum system, the form and position deviations are combined and the deviations at times appear to be significant (See FIG. 5).
It is not possible with the current datum system to distinguish a feature that has the correct form but is out of position from a feature that does not have the correct form. Without independent verification of both the form and position it is not possible to determine accurately the amount of distortion of the part. As positional deviation is often much larger than form deviation, combining the two results in an unacceptably large reported form tolerance during inspection. A part having an incorrect form can encompass any number of process induced form variations. Examples of process induced form variations include bulging of the shell due to shell weakness or shell creep, incorrect wax or metal shrink factor, and local shell strength conditions.
Consequently, there exists a need for a system and method that verifies the positional geometry of an airfoil independent of the feature geometry of the airfoil, root and shroud.
There also exists a need for a system and method that verifies the feature geometry of an airfoil independent of the positional geometry of the airfoil, root and shroud.
There further exists a need for a system and method that determines and accurately evaluates the positional deviations of an airfoil, root and shroud.